EUV experiments with helium nanodroplets

Helium nanodroplets are particularly attractive forms of matter for studying light-matter interaction in various regimes. They feature a homogeneous quantum liquid structure, a simple electronic structure of the constituent He atoms, and can be doped with atoms or molecules by the pick-up process to form extreme hetero-clusters. In my talk, I will present experimental results on the ignition of a helium nanoplasma by illuminating doped helium nanodroplets with ultrashort and intense NIR laser pulses. Surprisingly, we find a strong dependence on the dopant species, in particular for dopants which reside on the droplet surface instead of being immersed in the droplet interior. Using EUV radiation from a synchrotron and free-electron laser we recently studied the transition from the one-photon to collective ionization regimes. Dopant atoms are efficiently ionized either by charge-transfer of by energy-transfer from the helium depending on their position in / on the droplets.